Correction of coupled higher-order modes in the S-parameter characterization of wideband cavity beam position...

Wideband RF cavity beam position monitors (CBPM) have been increasingly employed for short bunch interval operations in several linear accelerators. At a few nanoseconds of bunch spacing, the loaded quality factor of the CBPM TM₁₁₀ dipole eigenmode is required to be sufficiently low to minimize the signal interference between consecutive bunches. Moreover, the bunched beam also couples to several unwanted higher-order modes (HOM), such as the TM₂₁₀ and TE₁₁₁ eigenmodes, which also may result in an error of the bunch position measurement if no precautions are taken. In the fabrication phase of CBPMs, the mode coupling can alter the TM₁₁₀ mode frequency and therefore causes an error in its tuning, e.g., 0.3% for a 4.875 GHz CBPM with Q_L = 22.5. This error needs to be identified so that the precise tuning is enabled since the dipole mode frequency becomes critical for the position evaluations as the signal processing is relatively phase-sensitive. This paper presents an approach to extract the pure unperturbed frequency of the dipole mode, not altered by the response to coupled HOMs. An analytic model based on the superposition law applied to electromagnetic fields has been developed to characterize the response of coupled HOMs in the S-parameter measurement. The model has been further verified with numerical simulation in the CST-Studio software by analyzing an approximate single-mode response CBPM. The correction method was applied on a wideband CBPM prototype for the pre-research plan of future high repetition rate hard XFEL at the Chinese Academy of Engineering Physics.